1. Field of the Invention
This invention relates to a hybrid scanning type of ion-implanting apparatus and ion-implanting method which magnetically sweep an ion beam and mechanically scan a target, and more particularly to means capable of sweeping an ion beam in a wide variety of energies and ion species and shortening an ion-implanting time to improve the throughput of the device. In this specification, the magnetic reciprocative scanning of the ion beam is referred to as xe2x80x9csweep or sweepingxe2x80x9d, and the mechanical reciprocative scanning of the target is referred to xe2x80x9cscan or scanningxe2x80x9d.
2. Description of the Related Art
Ion implantation, it is important to implant ions into a target (e.g. wafer) with good uniformity. This is particularly important when the ion implantation is adopted in a semiconductor manufacturing process. As the case may be, it is desired to irradiate the target with the ion beam scanned in parallel.
A conventional art of the ion-implanting apparatus proposed to fulfill the above-mentioned demand is shown in FIG. 2. This apparatus basically has the same structure as described in Japanese Patent Unexamined Publication No. Hei. 8-115701(JP-A-8-115701).
The ion-implanting apparatus includes an ion source 2 for drawing an ion beam 4, a mass separation magnet 6 for selecting a specific ion species drawn therefrom, an accelerator tube 8 for accelerating or decelerating the ion beam derived therefrom, a mass separation magnet 10 (also referred to as xe2x80x9cenergy separation magnetxe2x80x9d) for selecting the ion species with a specific energy from the ion beams 4 derived therefrom, a sweeping magnet 12 for sweeping the ion beam derived therefrom in an X-direction(for example, horizontal direction) under a magnetic field, and paralleling magnet 14 for bending the ion beam 4 derived therefrom again to scan the ion beam 4 in parallel in cooperation with the sweeping magnet, i.e. making the ion beam 4 in parallel to a Z-axis which is a movement direction of the ion beam 4. The ion species is defined by the mass number and the valence of the ion.
The ion beam 4 derived from the paralleling magnet 14 is applied to a target (e.g. wafer) 20 held in a holder 8 of a scan mechanism 16. Referring to FIG. 3, the scan mechanism 16 mechanically scans the target 20 within a sweep region 4a of the ion beam 4 in a Y-direction (e.g. vertical direction) perpendicular to the above X direction. Due to a cooperation between the scanning of the target 20 and the sweep of the ion beam 4, the ion can be implanted into the entire surface of the target 20 with good uniformity.
The ion-implantation in this ion-implanting apparatus can be controlled by a control circuit including an implanting control device 26 as shown in FIG. 4.
The desired ion species, beam energy, beam current (beam quantity) and implanting quantity are set by a man-machine interface 28 and supplied to an implanting control device 26.
The implanting control device 26 calculates the number of times of scanning of the target 20 for realizing the aimed implanting quantity by the aimed beam current on the basis of these items of set information, and controls the scan mechanism 16 through a scan control unit 32 to realize it. The scan mechanism 16 converts a control signal supplied from the scan control device 26 into a signal for driving a motor in the scan mechanism 16.
The initial value of the scan speed of the target 20 is constant, and made variable during implantation by the implanting control device 26 according to a change in the beam current of the ion beam 4 which is implanted. Specifically, as the beam current decreases, the scan speed is decreased. Inversely, as the beam current increases, the scan speed is increased. In order to implement this, the beam current IB of the ion beam which is being implanted is measured all the time by a dose Faraday 22 (see FIG. 3 also) arranged on the side of the upstream of the target 20. The measured beam current IB is supplied to the implanting control device 26 via a current converter 24.
The sweep current I(t) which drives the sweeping magnet 12 results from the shaping of a triangular wave. This waveform shaping is carried out by the implanting control device 26 through a known method described in Japanese Patent Unexamined Publication No. Hei. 9-55179(JP-A-9-55179). For example, using a multi-point beam monitor not shown arranged upstream and downstream side of the target 20, the beam current density distribution in the X direction on the target 20 is estimated, and the triangular wave is shaped so that the distribution approaches constant. Concretely, the triangular wave is shaped so that the sweep speed of the ion beam 4 is decreased at the position where the current density in the current density distribution is desired to be increased whereas the sweep speed is increased at the position where the current density is desired to be decreased, thereby forming a sweep signal S(t). The sweep signal is amplified by a driving amplifier 30 and is supplied to the sweeping magnet 12 as a sweep current I(t).
In the prior art ion-implanting apparatus, the sweep frequency of the ion beam 4 is fixed. The minimum number of times of scanning of the target 20 is also fixed in order to assure the constant uniformity of implanting.
More specifically, generally, assuming that the scan speed in the Y direction of the target 20 is fixed, the implanting uniformity for the target 20 is improved as the sweep frequency of the ion beam 4 becomes high. Namely, as described above, the scan speed of the target 20 is fixed when the beam current of the ion beam 4 is fixed. Therefore, when viewed from the target 20, as shown in FIG. 5, the ion beam is implanted while it draws the locus in zigzag. Therefore, it can be easily supposed that when the sweep frequency of the ion beam 4 becomes low, the area which was not implanted by scanning the target only once is produced.
In an actual implantation, as the case may be, drawing of the ion beam 4 stops momentarily because of discharging in a beam drawing portion (drawing electrode system) of an ion source 2. At this time, the change in the beam current IB of the ion beam 4 is measured by the dose Faraday 22 described above, and a command of stopping the scanning of the target 20 is issued from the implanting control device 26. This intends to prevent the area of the target not implanted from being generated. However, the change in the beam current IB is detected later than the real change by a delay time to (see FIG. 6) of one period at the maximum in the sweep of the ion beam 4. The reason is as follows. When the beam current IB changes at the instant the ion beam 4 passes the dose Faraday 22, this change cannot be measured by the dose Faraday 22 until the ion beam returns to the dose Faraday 22 again. This also applies to the detection at the time of recovery of the ion beam.
In addition, since the scan mechanism 16 has inertia, the scanning of the target 20 cannot be stopped within a zero time. At the time of the recovery of the ion beam 4, the scan speed cannot also be increased to a rated speed within a zero time. An example of the change in the scan speed of the target 20 during that time is shown in FIG. 6. In this example, the stopping time of the ion beam 4 is set for 0.1 sec.
For the above reason, when the drawing of the ion beam 4 is stopped for an instant, the non-uniformity of ion implantation in the Y-direction on the target occurs. An example of the distribution of the quantity of the implanted ion when the phenomenon shown in FIG. 6 occurs is shown in FIG. 7. It shows the example where the target 20 is scanned once, and discharge has occurred once when the ion beam 4 stands at the center of the target 20 in the Y-direction. In this example, the uniformity of implantation is lowered(deteriorated) to 1.156%.
Incidentally, it should be noted that the uniformity of implantation is represented by ("sgr"/M)xc3x97100[%] where the average value of the distribution of the quantity of the implanted ion is M and the standard deviation "sgr", and that of 0% is the best.
The uniformity of implantation is deteriorated as the sweep frequency of the ion beam 4 becomes low. This is because the above delay time to is prolonged correspondingly. The phenomenon of deterioration of the uniformity of implantation occurs likewise when the beam current IB of the ion beam 4 drawn from the ion source 2 varies.
In order to obviate such failure, the conventional ion-implanting apparatus flattens the dense/coarse portion of the above distribution of the implanting quantity by increasing the number of times of scanning of the target 20, thereby assuring prescribed uniformity of implantation. Namely, since the sweep frequency of the ion beam 4 is fixed, the conventional ion implantation apparatus is required to assure a constant minimum number of time of scan necessary to compensate for the above dense/coarse portion of the implanting quantity.
Therefore, although the beam current IB is sufficient when viewed from the ion source 2, the implantation time cannot be shortened since the minimum number of times of scan must be assured. In other words, the throughput of the apparatus cannot be improved.
When the sweep frequency of the ion beam 4 is increased, it cannot be increased excessively. The sweep frequency has a upper limit. This will be explained referring to FIG. 4. The sweep signal S(t) supplied from the implanting control device 26 is amplified by a driving amplifier 30 to provide a sweep current I(t) necessary to drive the sweeping magnet 12.
In such a configuration, the sweep frequency of the ion beam 4 is determined by the following factors.
(1) Magnitude of the sweep current I(t)
It depends on the ion species and energy of the ion beam 4. It also limited by the maximum output current of the driving amplifier 30.
(2) Magnitude of the output voltage V(t) from the driving amplifier 30
It is limited by the maximum output voltage from the driving amplifier 30.
(3) Inductance L of the coil 13 of the sweeping magnet 12
It is determined by the necessary magnetic flux density of the sweeping magnet 12.
Now, assuming that the sweep current when the ion species having a certain mass with certain energy is swept in a desired width by the sweeping magnet 12 is I(t), the output voltage at this time from the driving amplifier 30 is V(t), and the inductance of the coil 13 of the sweeping magnet 12 is L, with the resistance of the coil being negligible, the relationship among these items can be expressed by
V(t)=Lxc2x7dI(t)/dtxe2x80x83xe2x80x83[1]
An example of the relationship of the sweep current I(t) which is a triangular wave with the output voltage V(t) is shown in FIG. 8. The output voltage V(t) has a square waveform. If the sweep frequency of the ion beam 4, i.e. the frequency of the sweep current I(t) is increased twice for example, since the inductance L is a value intrinsic value to the sweeping magnet 12 the necessary output voltage V(t) is also increased twice as seen from Equation [1]. Further, when the sweep current I(t) is wave-shaped as described above in order to enhance the uniformity of implantation in the X-direction on the target, as understood from the example shown in FIGS. 9A and 9B, the output voltage V(t) which is larger (that corresponds to V1 in FIG. 9B) than that in the case of the triangular wave is required. This is because the voltage must be increased during the same period to increase dV(t)/dt. However, if this required V(t) exceeds the maximum output voltage of the driving amplifier 30, the rise in the sweep current I(t) slows down so that the uniformity of implantation in the X-direction is deteriorated.
On the other hand, the ion beam 4 having a lower energy and lighter ion species requires a smaller beam current I(t). This is because such an ion beam can be easily bent.
For the reason described above, in an actual apparatus, the upper limit of the sweep frequency of the ion beam 4 is limited by the energy of the ion beam 4, ion species and the maximum output voltage of the driving amplifier 30. Oppositely speaking, in the conventional ion implantation device, the sweep frequency of the ion beam 4 is fixed so that the energy of the ion beam 4 which can be swept properly and the ion species are limited.
Thus, for example, in order to deal with the ion beam 4 with higher energy, it is necessary to improve the specification such as boosting the maximum output voltage of the driving amplifier 30 or decreasing the inductance L of the coil 13 of the sweeping magnet 12. This requires serious cost and labor. This also applies to dealing with the heavier ion species.
Further, when the sweep frequency of the ion beam 4 is simply reduced in order to deal with the higher energy or heavier ion species, for the reason described above, if the minimum number of times of scanning of the target is not increased, the uniformity of implantation for the target 20 is deteriorated. However, if the minimum number of times of scan is increased, the time of implantation is lengthened so that the throughput of the device is deteriorated.
In view of the circumstance described, the main object of this invention is to provide an ion-implantation method and apparatus which can sweep the ion beam with a wide variety of energies and ion species without changing devices for sweeping the ion beam and shorten the implanting time to improve the throughput of the apparatus.
The ion-implanting method according to this invention comprising a step of changing a sweep frequency of the ion beam to be swept by said sweeping magnet according to at least one of the species and energy of the ion beam and changing the minimum number of times of scanning of the target to be scanned by said scan mechanism according to the changing of the sweep frequency.
The ion-implanting apparatus according to this invention comprises an implanting control device which serves to control sweeping an ion beam by a sweeping magnet and scanning a target by a scan mechanism, and has the functions of changing a sweep frequency of the ion beam to be swept by said sweeping magnet according to at least one of the species and energy of the ion beam and changing the minimum number of times of scanning of the target to be scanned by said scan mechanism according to the changing of the sweep frequency.
Since the sweep frequency of the ion beam is changed according to at least one of the ion species and energy of the ion beam to be swept, dI(t)/dt indicated in the Equation [1] can be varied. Therefore, even if at least one of the ion species and energy of the ion beam is changed, the ion beam can be swept normally within the capability of the devices relative to sweeping of the ion beam. Thus, without changing the devices relative to sweeping of the ion beam, the ion beam with a wide variety of energies and ion species can be swept normally.
Further, the ion beam to be swept having a lower energy and energy and lighter ion species can be more easily bent by the sweeping magnet. Therefore, the sweep current therefor may be reduced and the sweep frequency can be increased correspondingly. Thus, by increasing the sweep frequency, the minimum number of times of scanning of the target can be reduced to assure prescribed uniformity of implantation. Correspondingly, the implanting time can be shortened to improve the throughput of the apparatus.